1. Field of the Invention
The present invention relates to a photomask blank for making photomasks used in the fabrication of semiconductor integrated circuits, charge-coupled devices (CCD), color filters for liquid crystal displays (LCD), magnetic heads and the like.
2. Prior Art
In semiconductor fabrication, the increasing level of integration in large-scale integrated circuits in particular has led to a growing need in recent years for circuit patterns of smaller geometries. This need has intensified the desire for technology to reduce the size of features in the wiring patterns that make up a circuit and technology to reduce the size of features in contact hole patterns for the interlayer connections that make up a cell. Accordingly, in the production of photomasks onto which circuit patterns have been written and which are used in photolithography to form such wiring patterns and contact hole patterns, there exists a need for a technology which, in keeping with the above trend toward smaller geometries, is capable of writing circuit patterns to even smaller dimensions and a higher accuracy.
To create a photomask pattern of even higher accuracy on a photomask substrate, it is first necessary to form a highly accurate resist pattern on the photomask blank. Because photolithography during the processing of an actual semiconductor substrate involves carrying out reduction projection, the photomask pattern is about four times larger than the size of the pattern actually required. However, this does not mean that the need for accuracy can be relaxed accordingly. Indeed, the photomask serving as the master must have a level of accuracy that is higher than that required of the pattern after exposure.
Moreover, in lithography as it is already currently practiced, the dimensions of the circuit pattern to be written are substantially smaller than the wavelength of the light used for exposure. Hence, when a photomask pattern bearing a circuit shape that has been enlarged four-fold is used, owing to such effects as the light interference that arises during actual photolithography, the photomask pattern is not transferred in the same exact shape to the resist film. To reduce such effects, it is sometimes necessary to carry out processing which gives the photomask pattern a shape that is more complex than the actual circuit pattern—that is, a shape obtained by the application of, for example, optical proximity correction (OPC). Accordingly, in lithographic technology for patterning photomasks as well, there exists a desire today for processing methods of even higher accuracy. The lithographic performance is sometimes referred to as the “minimum resolution.” It is desired that the lithographic technology used in photomask-making operations have a resolution limit which represents an accuracy that is substantially equal to or greater than that of the resolution limit required in the photolithography employed in semiconductor fabrication where the photomask is used.
Typically, in the formation of a photomask pattern, a layer of photoresist is deposited on a photomask blank composed of a transparent substrate having a light-shielding film thereon. A pattern is written onto the photoresist film by exposure to electron beams and development is carried out, thereby giving a resist pattern. Next, the light-shielding film is etched using the resist pattern as the etching mask, thereby forming a light-shielding pattern. When a light-shielding pattern of smaller minimum feature size is to be formed, if processing is attempted with a resist film of about the same thickness as that used prior to scaling down of the dimensions, the ratio of the film thickness to the size of pattern features, i.e., the aspect ratio, increases and the resist pattern shape degrades, preventing pattern transfer from being properly carried out. In some cases, the resist pattern may even collapse or separate from the substrate. It is thus necessary to reduce the thickness of the resist film in keeping with such reductions in feature size.
As for the light-shielding film material that is etched using a resist as the etching mask, although numerous materials have hitherto been described in the art, chromium compound films are almost always used in practice, owing to the extensive knowledge that exists concerning the etching of such films and the fact that the standard processing operations are well-established.
However, the oxygen-containing chlorine-based dry etch commonly used for dry etching chrome-based films such as chromium compound films often also etches organic films to some degree. As a result, when such etching is carried out using a thin resist film, it is difficult to accurately transfer the resist pattern. How to require of a resist that it have an etch resistance which enables etching to be carried out to a high resolution and to a high accuracy has been a daunting challenge. Hence, to achieve both a high resolution and a high accuracy, light-shielding film materials need to be reexamined so that a change can be made from an approach which relies exclusively on resist performance to an approach which also enhances the performance of the light-shielding film.
As one way of addressing this problem, the inventors have earlier found that metal silicide-based materials provide a higher light-shielding performance per unit film thickness than chrome-based materials, particularly at exposure wavelengths below 250 nm (see JP-A 2006-78807). They have also shown that, although transition metal silicide-based materials, when placed in a low oxidation state, are at risk of having a diminished chemical stability to ammonia/hydrogen peroxide aqueous solution or the like commonly used in cleaning, by setting the proportion of transition metal (transition metal/(transition metal+silicon)) in such a material to 20% or less, the material can be assured of having the necessary chemical stability at the time of photomask fabrication. In addition, the inventors have reported that, when a chrome-based material is used to form an antireflective film, differences in the etch selectivities of the metal silicide-based material and the chrome-based material enable mutually selective etching to be carried out. That is, by using a chrome-based material film as a hardmask, the load on the resist film that is initially deposited decreases, enabling high-precision etching to be carried out even at a lower resist film thickness.
The use of e-beam lithography in the production of photomasks is well known. Although e-beam lithography, when used to repeatedly write the same pattern, does have a lower efficiency than photolithography with a mask, it is advantageous for forming patterns of smaller geometry and is thus better suited for photomask production than photolithography. Attempts have been made recently to increase the current density during electron beam irradiation, the goal being to improve the e-beam exposure system and raise throughput. However, there is a possibility that increasing the current density at this time will tend to give rise to a “charge-up” effect during e-beam writing.
As noted above, the inventors earlier disclosed the use of transition metal silicide compounds instead of conventional chrome-based materials as a more advantageous light-shielding film material in photomasks adapted for use with low-wavelength exposure light. However, the chemical stability desired of the light-shielding film material in a photomask makes it necessary to ensure that the silicon content, expressed as silicon/(silicon+transition metal), exceed 80%. Yet, the desire for a material which is less prone to charge-up will most likely give rise to a need for an electrical conductivity that is higher than the range which has hitherto been required. The inventors anticipated that, in order to obtain a light-shielding film made with a transition metal silicide compound that achieves a comparable functionality using light having a wavelength of 250 nm or less, it would be possible to increase the conductivity by raising the transition metal concentration. However, given the existing knowledge that there is no assurance of chemical stability when a light-shielding film material having a silicon content of 80% or less is used, it became apparent that a new approach would be needed to ensure the chemical stability.